Electrolytic phosphate chemical treatment method

ABSTRACT

The object of the present invention is to provide an electrolytic phosphate chemical treatment method capable of improving the reaction efficiency on a metal surface (interface) by preventing the reaction in the solution phase so as to reliably prevent sludge formation during continuous treatment.  
     The present invention relates to a method of forming a film composed of a phosphate compound and a metal on the surface of an article to be treated by performing electrolytic treatment on a metal material article to be treated in a phosphate chemical treatment bath by contacting said metal material having electrical conductivity with said phosphate chemical treatment bath containing phosphate ions and phosphoric acid, nitrate ions, metal ions that form a complex with phosphate ions in said phosphate chemical treatment bath, and metal ions for which the dissolution-precipitation equilibrium potential at which ions dissolved in said phosphate chemical treatment bath are reduced and precipitate as metal is equal to or greater than −830 mV, which is the cathodic reaction decomposition potential of the solvent in the form of water when indicated as the hydrogen standard electrode potential, and is substantially free of metal ions other than those which are a component of the film; wherein the ORP (oxidation-reduction potential) of said phosphate chemical treatment bath (indicated as the potential relative to a standard hydrogen electrode) is maintained at equal to or greater than 700 mV.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to surface treatment of a metal,and more particularly, to surface treatment of a metal using a phosphatechemical film.

[0003] 2. Description of the Related Art

[0004] To begin with, if phosphate chemical treatment technology were tobe divided into electrolytic treatment and non-electrolytic treatment,electrolytic treatment would be a new technology while non-electrolytictreatment would be a conventional technology. Although the reaction ofphosphate chemical treatment is an electrochemical reaction for bothnon-electrolytic treatment and electrolytic treatment, the contents ofthat reaction are quite different.

[0005] The inventor of the present invention previously filed a patentrelating to electrolytic phosphate chemical treatment (JapaneseUnexamined Patent Publication No. 2000-234200). At the time of theprevious filing, a study was conducted relating to electrolyticphosphate chemical treatment of the prior art. However, studiesregarding the inherent prior art of non-electrolytic phosphate chemicaltreatment were not always adequate. To begin with, the differencebetween the electrochemical reactions of non-electrolytic treatment andelectrolytic treatment are clarified with respect to surface treatment.To accomplish this object, the mechanism of the chemical reaction innon-electrolytic treatment is shown in FIG. 8. In contrast, themechanism of the electrochemical reaction in electrolytic treatment isshown in FIG. 1.

[0006] The major differences between non-electrolytic treatment andelectrolytic treatment with respect to surface treatment can besummarized as indicated below.

[0007] (i) In the case of non-electrolytic treatment, a film is formedby an electrochemical reaction in the same treatment bath and on thesame metal surface. Namely, the anode and cathode in the electrochemicalreaction are the same metal surfaces. On the other hand, electrolytictreatment involves the application of voltage and current from anexternal power supply in the same treatment bath. A film is then formedby an electrochemical reaction under conditions in which the electrodesare divided into an anode and cathode. Consequently, the electrochemicalreaction in electrolytic treatment is divided into a reaction on ananode and a reaction on a cathode that are separated in a treatmentbath.

[0008] (ii) In electrolytic treatment, as shown in FIG. 1, a solution isdivided into a solution phase and an interface (metal surface). It isnecessary that the applied voltage and current be limited to acting onlyon the interface. As a result, the film forming reaction of the solutioncomponent due to electrolysis only acts on the metal surface. In thismanner, the phase transition (film formation) from the liquid to solid,which constitutes the deposition of the film, can be limited to only themetal surface. In other words, in electrolytic treatment, it isimportant to create a mechanism that is capable of preventing a reactionin the solution phase.

[0009] On the other hand, in non-electrolytic treatment, although filmformation occurs on the surface of an article to be treated, thereaction components are supplied to a location away from the metalsurface (solution phase). Namely, in non-electrolytic treatment, a filmis formed on the metal surface by allowing the component of the solutionphase to react. This is because film formation (phase transition from aliquid to a solid) is carried out more easily on the surface of thearticle to be treated (metal) than in the solution phase. Consequently,it is not necessary in non-electrolytic treatment to strictly separatethe solution phase and interface as compared with electrolytictreatment. From the standpoint of forming a film by controlling anelectrochemical reaction, there is a considerable difference betweenforming sludge by reacting the component of a solution phase and notforming sludge by not allowing to react.

[0010] (iii) Difference in Reaction Voltage

[0011] The present invention is targeted at film formation from anaqueous solution using water as the solvent. The electrochemicalreaction in non-electrolytic treatment does not assume the decompositionof a solvent in the form of water. Consequently, the electrochemicalreaction is at a voltage of 1.23 V or less, the decomposition voltage ofwater. On the other hand, in the case of electrolytic treatment, whichuses an external power supply, it is typically accompanied by adecomposition reaction of water (solvent). Consequently, theelectrolytic reaction voltage typically exceeds 1.23 V. This differencein the reaction voltage, along with the presence or absence of theaccompanying decomposition of solvent (water), are the major differencesbetween electrolytic treatment and non-electrolytic treatment.

[0012] Next, an explanation is provided of the prior art with respect toelectrolytic treatment.

[0013] As an example of the prior art, Japanese Unexamined PatentPublication No. 2000-234200 discloses an electrolytic phosphatetreatment method comprising:

[0014] forming a film containing a phosphate compound and a metal thatis not a phosphate on the surface of an article to be treated havingelectrical conductivity by performing electrolytic treatment bycontacting the article to be treated with a phosphate chemical treatmentbath containing phosphate ions and phosphoric acid, nitrate ions, metalions that form a complex with the phosphate ions in the phosphatechemical treatment bath (e.g., zinc, iron, manganese or calcium ions),and metal ions for which the electrical potential at which the ionsdissolved in the phosphate chemical treatment bath are reduced andprecipitate as metal is equal to or greater than the cathodicelectrolysis reaction potential of the solvent in the form of water orequal to or greater than −830 mV (e.g., nickel, copper or iron ion)based on a reference electrode potential; wherein

[0015] the above phosphate chemical treatment bath contains 0-400 ppm ofmetal ions other those which are a component that forms the above film(e.g., sodium ion), and is substantially free of solids (sludge) havingan effect on the film formation reaction; and

[0016] the above article to be treated is treated by electrolysis in theabove phosphate chemical treatment bath with a metal material that formsa complex with phosphate ions in this treatment bath, and a metalmaterial for which the electrical potential at which ions thereofdissolved in the phosphate chemical treatment bath are reduced andprecipitate as metal is, based on a reference electrode potential, equalto or greater than the cathodic electrolysis reaction potential of thesolvent in the form of water or −830 mV or higher (indicated as thepotential relative to a standard hydrogen electrode), and/or aninsoluble electrode material.

[0017] This electrolytic phosphate treatment method of the prior art wasdeveloped in order to efficiently form a phosphate-metal mixed chemicalfilm without causing the formation of sludge in the treatment bath.However, when this method is used to carry out treatment continuously,it was found that sludge forms depending on the treatment conditions.

[0018] One of the reasons for being unable to practically apply theelectrolytic phosphate chemical treatment in Japanese Unexamined PatentPublication No. 2000-234200 is that in phosphate chemical treatment, allthree constituent features relating to electrolytic treatment consistingof the solution, counter electrode and article to be treated areinvolved in the reaction. The following Table 1 is shown in reference tothis point. TABLE 1 Classification of Wet Electrolytic Treatment (O:Reacts, X: Does not react) Counter Article to be electrode Solutiontreated Electroplating O X X Electrodeposition X O X coatingElectrolytic phosphate O or X O O chemical treatment

[0019] In the electrolytic phosphate chemical treatment of theabove-mentioned Japanese Unexamined Patent Publication No. 2000-234200,attention was not paid to “not allowing the components in solution toreact at a location other than the electrode surface” in particular.Consequently, corrective actions and accommodations were performedconsisting of:

[0020] (1) prevention of contamination by impurities (Na ions, etc.)

[0021] (2) prevention of self-decomposition and aggregation of solutioncomponents by constantly filtering and circulating the treatment,maintaining the temperature and so forth, and

[0022] (3) use of a complex.

[0023] However, in the case of performing treatment continuously, it wasfound to be difficult to maintain “not allowing the components insolution to react at a location other than the electrode surface” withonly the accommodations made in the above-mentioned invention ofJapanese Unexamined Patent Publication No. 2000-234200. Namely, inJapanese Unexamined Patent Publication No. 2000-234200, although thetreatment bath is constantly filtered and circulated during electrolytictreatment, it was found that solids (sludge) are trapped by the filterat that time. The amount captured can be maintained within a range thatcan be allowed with respect to film formation in terms of practicalapplication of this method. However, this sludge becomes partiallyredissolved (for example, Zn₂Fe(PO₄)₂+6H⁺→2H₃PO₄+2Zn²⁺+Fe²⁺). Thisphenomenon (reaction) impairs film formation. Thus, it is thought to benecessary to devise even more effective countermeasures in order tostabilize the electrolytic phosphate chemical treatment bath and preventthe formation of a waste product in the form of sludge.

[0024] As has been described above, the prior art relating toelectrolytic phosphate chemical treatment was inadequate with respect tonot allowing the reaction of solution phase components (not allowing theformation of sludge), which is the basis of electrolytic surfacetreatment technology. For this reason, the electrolytic phosphatechemical treatment technology of the prior art was inadequate as anelectrolytic surface treatment technology.

SUMMARY OF THE INVENTION

[0025] The object to be solved by the present invention is to assemblean electrolytic phosphate chemical treatment technology in the form of atechnology that is in accordance with the general principle ofelectrolytic surface treatment. That is, to limit the electrolyticphosphate chemical treatment reaction to only a reaction of a metal(electrode) surface, and not a liquid phase reaction.

[0026] Although the inventor of the present invention devised acountermeasure for preventing an electrolytic reaction in the solutionphase in an invention disclosed in previously disclosed JapaneseUnexamined Patent Publication No. 2000-234200, this could not always besaid to be adequate with respect to reliably preventing the solutionphase reaction and limiting to only a reaction of a metal surface.Therefore, the problem to be solved by the present invention is toimprove the level of control of an electrolytic phosphate chemicaltreatment reaction as an electrolytic surface treatment in the inventiondisclosed in Japanese Unexamined Patent Publication No. 2000-234200.Namely, the object of the present invention is to establish a means forfurther improving the reaction efficiency on a metal surface (interface)by preventing the reaction in the solution phase to reliably preventsludge formation during continuous treatment.

[0027] According to a first mode of the present invention, the presentinvention is an electrolytic phosphate chemical treatment method offorming a film composed of a phosphate compound and a metal that isreduced and precipitated from an ionic state on the surface of a metalmaterial article to be treated by performing electrolytic treatment onsaid article to be treated in a phosphate chemical treatment bath bycontacting said metal material having electrical conductivity with saidphosphate chemical treatment bath containing phosphate ions andphosphoric acid, nitrate ions, metal ions that form a complex withphosphate ions in said phosphate chemical treatment bath, and metal ionsfor which the dissolution-precipitation equilibrium potential at whichions dissolved in said phosphate chemical treatment bath are reduced andprecipitate as metal is equal to or greater than −830 mV, which is thecathodic reaction decomposition potential of the solvent in the form ofwater when indicated as the hydrogen standard electrode potential, andis substantially free of metal ions other than those which are acomponent of the film; wherein,

[0028] the ORP (oxidation-reduction potential) of said phosphatechemical treatment bath (indicated as the potential relative to astandard hydrogen electrode) is maintained at equal to or greater than700 mV.

[0029] The above “substantially free of metal ions other than thosewhich are a component of the film” means that the content of metal ionsother than those which are a component of the film is either zero or 0.5g/L or less.

[0030] In this manner, by making the ORP equal to or greater than 700mV, the sludge formation of the electrolytic treatment bath of thepresent invention can be made to be substantially zero.

[0031] According to a second mode of the present invention, the aboveelectrolytic treatment preferably uses for the electrode material thatdissolves in the treatment bath a metal that forms a complex withphosphoric acid and phosphate ions in the phosphate chemical treatmentbath and/or a metal material for which the dissolution-precipitationequilibrium potential at which ions dissolved in the phosphate chemicaltreatment bath are reduced and precipitate as metal is greater than orequal to −830 mv, which is the cathodic reaction decomposition potentialof the solvent in the form of water when indicated as the hydrogenstandard electrode potential, and a metal material that is insolubleduring electrolysis.

[0032] According to a third mode of the present invention, it ispreferable to control the amount of Fe ions dissolved into the treatmentbath from an Fe electrode in the case of using an Fe electrode as theelectrode that dissolves in the treatment bath during cathodic treatmentof the above article to be treated in order to make the above ORP of thephosphate chemical treatment bath equal to or greater than 700 mV.

[0033] Moreover, according to a fourth mode of the present invention, itis preferable to control the amount of Fe ions dissolved into thetreatment bath in anodic treatment in which the article to be treated isa steel material and the steel material in the form of the article to betreated is dissolved as the anode, and the amount of Fe ions thatdissolve in the treatment bath in the case of using an Fe electrode incathodic treatment, so that the above ORP of the phosphate chemicaltreatment bath is equal to or greater than 700 mV.

[0034] In addition, according to a fifth mode of the present invention,it is preferable that a chemical that contains Fe ions which replenishthe above phosphate chemical treatment bath be a Fe-phosphate complex inorder to make the above ORP of the phosphate chemical treatment bathequal to or greater than 700 mV.

[0035] According to a sixth mode of the present invention, the above ORPof the phosphate chemical treatment bath is preferably equal to orgreater than 770 mV.

[0036] Moreover, according to a seventh mode of the present invention,metal ions that form a complex with phosphoric acid and phosphate ionsin the phosphate chemical treatment bath are preferably at least onetype of Zn, Fe, Mn or Ca ions.

[0037] In addition, according to an eighth mode of the presentinvention, an electrolytic phosphate chemical treatment method ispreferable which removes gases generated and dissolved in anelectrolytic treatment tank in the form of NO, NO₂ and/or N₂O₄ from thebath by separating the treatment tank into an electrolytic treatmenttank that carries out electrolytic treatment and an auxiliary tank thatdoes not carry out electrolytic treatment, circulating the treatmentbath between the two tanks, and providing a mechanism that openstreatment liquid to the atmosphere either between the above two tanks orwithin the two tanks, as a means of separating NO₂, N₂O₄ and/or NO gasformed in the treatment bath accompanying electrolytic treatment fromthe treatment bath.

[0038] According to a ninth mode of the present invention, the aboveauxiliary tank that does not carry out electrolytic treatment has amechanism in which treatment liquid is passed through a permeable solidstructure such as a film, and a filter having a filtering mechanism ispreferably used for such an auxiliary tank.

[0039] Moreover, according to a tenth mode of the present invention, aliquid circulation circuit is preferably provided that removes a portionof the treatment liquid at a location prior to being introduced into thefilter material in the filter, exposes the removed treatment liquid tothe atmosphere, and returns it to the electrolysis tank after removinggases in the form of nitrogen oxides present in the treatment liquid.

[0040] According to an eleventh mode of the present invention, the aboveORP of the treatment bath is preferably equal to or greater than 840 mV.

[0041] Moreover, according to a twelfth mode of the present invention,it is preferable to maintain the above treatment bath in a constantstate by measuring the above ORP value of the treatment bath andchanging the amount and/or composition of replenishing chemicalcorresponding to the change in that value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a drawing showing the mechanism of the electrochemicalreactions in electrolytic treatment.

[0043]FIG. 2 is a drawing showing the constituent features ofelectrolytic treatment used in the examples and comparative examples.

[0044]FIG. 3 is a perspective view showing an overview of electrolytictreatment used in the examples and comparative examples.

[0045]FIG. 4 is a perspective view of an article to be treated in theform of a stator housing used in the examples and comparative examples.

[0046]FIG. 5 is a graph showing the schedule of electrolytic treatmentcarried out in the examples and comparative examples.

[0047]FIG. 6 is a block drawing of open system lines showing a firstmode for carrying out the present invention.

[0048]FIG. 7 is a block drawing of closed system lines showing a firstmode for carrying out the present invention.

[0049]FIG. 8 is a drawing showing the mechanism of the electrochemicalreactions in non-electrolytic treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] The potential difference distribution of an electrolytic reactionrelating to surface treatment using an external power supply is as shownin FIG. 1 between two electrodes (namely an anode and cathode (workingelectrode)). In FIG. 1, when a voltage is applied between the twoelectrodes, the voltage distribution is divided into two parts as shownin this drawing. Namely, the voltage between the two electrodes isdivided into a potential difference at the electrode interface and apotential difference in the solution phase.

[0051] Film formation in electrolytic treatment is carried out bycausing the components dissolved in the solution to undergo anelectrochemical reaction (oxidation reaction or reduction reaction) onthe electrode (solid) surface due to this change in potential differenceat the electrode interface. Namely, a film is formed by a reaction(interface reaction) at the electrode surface (interface).

[0052] On the other hand, the change in the potential difference in thesolution phase occurs as a result of a chemical reaction accompanying achange in the potential difference at the electrode interface, and is areflection of the electrochemical equilibrium between chemical componentions in the solution phase. Namely, changes in potential difference inthe solution phase do not reflect a chemical reaction resulting fromelectrolysis of solution phase components. Consequently, it is essentialthat changes in the potential difference in the solution phase be of anextremely low voltage and do not cause a phase transition (solution →solid) accompanying chemical reaction. Namely, in electrolytic surfacetreatment, it is necessary that the electrolytic treatment reaction notbe allowed occur in the solution phase.

[0053] On the basis of the above, in electrolytic surface treatmentpertaining to film formation, a solution phase reaction is a detrimentalreaction. In electrolytic phosphate chemical treatment, sludge forms ifa solution phase reaction occurs. Electrolytic surface treatment that isalready used practically (electroplating, electrodeposition coating)employs contrivances such that only the interface reaction is allowed tooccur while the solution phase reaction is not. Namely, actions aretaken so that all of the electrical energy (voltage, current) applied tothe electrolytic treatment system acts only on the electrode interface.

[0054] The object of the present invention is to improve the efficiencyof the electrolytic phosphate chemical treatment reaction. The means forachieving this is basically similar to other electrolytic surfacetreatment, consisting of preventing a reaction in the solution phase(solution phase reaction) and improving the efficiency of the reaction(interface reaction) at the electrode surface (interface). However, ameans that is unique to electrolytic phosphate chemical treatment isrequired for the specific means for achieving this.

[0055] Namely, a first means is preventing the reaction in the solutionphase (solution phase reaction).

[0056] In the case of electroplating, which is an electrolytic surfacetreatment that is already used practically, metal ions that havedissolved from the anode are present in solution as a complex, and arestable in the solution. The reasons for using a cyanide-complex for theelectroplating bath is that cyanide complex can be used that is stablein the solution phase with respect to application of voltage. As aresult, voltage applied between the electrodes does not act in thesolution phase. The change in potential difference of the appliedvoltage only acts at the electrode surface, while the metal to be plateddissolves at the anode and precipitates at the cathode.

[0057] In cationic electrodeposition coating, which is anotherelectrolytic surface treatment that is used practically, the solutecomponent is an organic substance, and a complex cannot be used in themanner of electroplating. Consequently, accommodations must be madeusing a different method.

[0058] The electrodeposition coating liquid is a solution in which anorganic substance is dispersed. Moreover, the anode in cationicelectrodeposition coating is insoluble. In the case of electrodepositioncoating, preventing the solution phase reaction means maintaining thecoating liquid in a state in which organic substances are dispersed. Ifthe coating liquid is unable to be maintained in a state in whichorganic substances are dispersed, the coating liquid aggregatesresulting in the formation of solids. Namely, the solution phasereaction proceeds.

[0059] Actions are taken for electrodeposition coating so that asolution state can be maintained at all times. More specifically, theseactions consist of controlling the temperature at a constanttemperature, preventing contamination by Na ions and other impurities,and constantly filtering and circulating the coating liquid to preventthe decomposition and separation of organic substances of the solutioncomponents, including solids. Since these actions are taken,electrodeposition coating is able to maintain a solution state at alltimes and prevent reactions in the solution phase. When a voltage isapplied between electrodes of an electrolysis liquid controlled in thismanner, that voltage does not act in the solution phase. Changes in thepotential difference of the applied voltage act only at the electrodesurface, and an electrodeposition coating film precipitates on thesurface of the cathode (working surface).

[0060] Namely, in practical electrolytic treatment that forms a film,means for preventing a reaction in the solution phase of the above FIG.1 are determined and strictly observed.

[0061] In the electrolytic phosphate chemical treatment of the priorart, the above approach of preventing reaction in the solution phase wasnot given adequate consideration at the practical level. Thoseaccommodations are made in the present invention.

[0062] Next, a second means of improving electrolytic phosphate chemicaltreatment reaction efficiency consists of improving the reactionefficiency at the electrode surface (interface).

[0063] Although electrolytic phosphate treatment involves electrolyticsurface treatment using water for the solvent, the following clarifiesdifferences with other electrolytic treatment (such as electroplatingand electrodeposition coating) that similarly use water for the solvent.

[0064] In electrolytic phosphate chemical treatment (cathodictreatment), the gas that is generated from the treatment bath differsfrom conventional electrolytic treatment (e.g., electroplating andelectrodeposition coating). This is illustrated in Table 2. TABLE 2Electrolytic Treatment and Reaction Components Solvent (water) SoluteHydrogen Oxygen gas Film Non-film gas (H₂) (O₂) components componentsElectro- O O (formed) O (formed) X (not plating (formed) formed)Electro- O O (formed) O (formed) X (not deposition (formed) formed)coating Electrolytic O O (formed) O (formed) O (formed: phosphate(formed) nitrogen chemical oxides) treatment

[0065] In the case of conventional electrolytic treatment using waterfor the solvent, the gas that is generated from the treatment bath isonly hydrogen gas and oxygen gas resulting from electrolysis of water.However, in the case of electrolytic phosphate chemical treatment, inaddition to hydrogen and oxygen, there are also nitrogen oxidesgenerated by decomposition of NO₃ ⁻, a solute component. As shown inTable 3, the states of these nitrogen oxides consist of NO, NO₂ andN₂O₄, and their boiling points at atmospheric pressure differconsiderably. TABLE 3 Differences in Boiling Points at AtmosphericPressure NO: −151° C. NO₂: 21.15° C. H₂: −252° C. N₂O₄: 29.07° C. O₂:−182° C.

[0066] Thus, if the state of the nitrogen oxides generated iscontrolled, the reaction state in the treatment bath is presumed tochange considerably. This was not examined at all in Japanese UnexaminedPatent Publication No. 2000-234200.

[0067] Table 3 shows a comparison of the boiling points of each gas atatmospheric pressure. In the case of conventional electrolytic surfacetreatment (electroplating and electrodeposition coating), the gasgenerated in the electrolysis reaction consists only of hydrogen gas andoxygen gas as a result of electrolysis of the solvent in the form ofwater as shown in Table 2. The boiling points of hydrogen and oxygen areextremely low as shown in Table 3. This indicates that both hydrogen andoxygen are easily evaporated and removed from the treatment bath.

[0068] However, the gases generated in electrolytic phosphate chemicaltreatment consist of nitrogen oxide gas (N₂O₄, NO₂ and NO) in additionto hydrogen gas and oxygen gas as shown in Table 2. It is clear that theease by which this gas is removed from the treatment bath differsdepending on the state of this nitrogen oxide gas (N₂O₄, NO₂ and NO).Namely, whether the nitrogen oxide gas generated is in the form of N₂O₄and NO₂ or NO results in a considerable difference in the conditions bywhich the gas is removed from the treatment bath. If the gas that isgenerated can be limited only to NO, the reaction (interface reaction)at the electrode surface (interface) is thought to be able to bemaintained at the level of electroplating. However, if the gas that isgenerated contains N₂O₄ and NO₂, that gas cannot be easily removed fromthe treatment bath, and it is therefore presumed that the reactionefficiency at the electrode surface (interface) would decrease.

[0069] A decrease in the reaction efficiency at the electrode surface(interface) is presumed to cause a decrease in adherence between thefilm and article to be treated. Thus, limiting the gas generated to NOonly is required for electrolytic phosphate chemical treatment, and thepresent invention provides a specific method for accomplishing this.

[0070] Elementary Reaction of Electrolytic Phosphate Chemical TreatmentReaction and Prevention of Solution Phase

[0071] Reaction

[0072] Possible elementary reactions that may take place in electrolyticphosphate chemical treatment are shown in Tables 4 and 5.

[0073] The following provides an explanation of specific measures forpreventing the solution phase reaction.

[0074] As shown in FIG. 1, the solution phase reaction is not affectedby the application of voltage and current by an external power supply inthe case of fundamental electrolytic surface treatment. This should alsobe observed in electrolytic phosphate chemical treatment as well.However, conventional non-electrolytic phosphate chemical treatmentforms a film by using a solution phase reaction (see FIG. 8).

[0075] Electrochemical equilibrium reactions that have the possibilityof occurring in the solution phase of an electrolytic phosphate chemicaltreatment bath are shown in Table 4. TABLE 4 Electrochemical EquilibriumReactions that can Occur in the Solution Phase Dissociation of H₃PO₄ →H⁺ + H₂PO₄ ⁻ (1) phosphoric acid H₂PO₄ ⁻ → 2H⁺ + PO₄ ³⁻ (2) Fe²⁺/Fe³⁺Fe²⁺ → Fe³⁺ + e⁻ (0.77 V) (3)

[0076] The reactions of (1) through (3) in Table 4 are essentialreactions in non-electrolytic treatment, and they take place in thesolution phase in non-electrolytic treatment.

[0077] The reactions of (1) through (3) are reactions that occur innon-electrolytic treatment. This means that the reactions of (1) through(3) occur due to factors other than the application of voltage andcurrent to the treatment bath. Namely, they occur due to changes in theelectrochemical conditions (pH, ORP, etc.) of the treatment bath. Thus,the electrochemical conditions of the treatment bath can be set toconditions under which the reactions of (1) through (3) do not proceed,in order to prevent the reactions of (1) through (3).

[0078] Next, an explanation is provided of the conditions under whichthe above reactions of (1) through (3) occur in the solution phase,along with their detrimental effects.

[0079] (i) Dissociation of Phosphoric Acid

[0080] When dissociation of phosphoric acid (H₃PO₄ → H₂PO₄ ⁻→ PO₄ ³⁻)progresses in the solution phase of the treatment bath, it becomesimpossible for phosphate ions to dissolve and exist in the treatmentbath, resulting in the formation of sludge (Zn₂Fe(PO₄)₂, M(PO₄)). Thedissociation state of phosphoric acid in a non-electrolytic treatmentbath is between H₃PO₄ and H₂PO₄ ⁻. The degree of dissociation of H₃PO₄ →H₂PO₄ ⁻ can be expressed as the orthophosphoric acid ratio (H₃PO₄/H₂PO₄⁻). The following provides an explanation of the relationship between pHand orthophosphoric acid ratio. Although the orthophosphoric acid ratiois 1 when the pH is 0, it is roughly 0.1 at pH 3 (see Ohki, M. andTanaka, M. ed., Iwanami Koza Publishing, Modern Chemistry 9, Oxidationand Reduction of Acids and Bases, 1979, p. 75). Namely, theorthophosphoric acid ratio (H₃PO₄/H₂PO₄ ⁻) decreases from 1 to 0.1 asthe pH changes from 0 to 3.

[0081] As was previously mentioned, non-electrolytic treatment involvesthe formation of a film by reacting components in solution. Filmformation takes place by dissociating phosphate ion to PO₄ ³⁻ andreacting with film forming metal ions (e.g., zinc ions). Consequently,in a non-electrolytic treatment bath, the composition consists mainly ofH₂PO₄ ⁻ to facilitate progression of dissociation of phosphate ions.Consequently, a bath consisting primarily of H₃PO₄ at pH 2.5 or lowerdoes not allow the formation of a film in non-electrolytic treatment.For this reason, the pH of a non-electrolytic treatment bath is roughly3, and H₃PO₄/H₂PO₄ ⁻ is controlled in the form of an acid ratio.

[0082] The use of a treatment bath roughly at pH 3 for thenon-electrolytic treatment bath indicates that there is a possibility ofsludge forming easily if electrolytic treatment is simply carried out atthat pH.

[0083] In the present invention, it is essential to not allow theformation of sludge. In order to not allow sludge to be formed in thetreatment bath, it is necessary to control the dissociation state ofphosphoric acid with the pH. More specifically, the pH of theelectrolytic treatment bath is 2.5 or lower, and more preferably pH 2 orlower.

[0084] Although a pH of 0.5 to 5 was used in the prior art (JapaneseUnexamined Patent Publication No. 2000-234200), in the presentinvention, it is preferable that the pH be 2.5 or lower. This isbecause, if the pH of the treatment bath exceeds 2.5, the ratio of metalions such as Zn and Mn, which form phosphate compounds by bonding withphosphate ions, to phosphoric acid (ions) becomes relatively large,thereby facilitating the formation of sludge.

[0085] (ii) Reaction Accompanying Decrease in Solubility of Fe Ions dueto Fe²⁺ → Fe³⁺

[0086] Fe ions dissolve in the treatment bath when a steel material isused as the article to be treated and when an Fe electrode is used forthe film forming metal electrode in electrolytic chemical treatment. Thedissolution of Fe proceeds in the manner of Fe → Fe²⁺ → Fe³⁺, anddissolves and exists in the treatment bath in the state of Fe²⁺ or Fe³⁺.

[0087] As the reaction of Fe²⁺ → Fe³⁺+e⁻ proceeds, the solubility of Feions decreases and sludge forms. The reaction of Fe²⁺ → Fe³⁺+e⁻ of (0.77V) of formula (3) means that Fe ions can proceed in the dissolved stateof Fe²⁺ or Fe³⁺ in solution only when the ORP (oxidation-reductionpotential; hydrogen standard electrode potential) of the treatment bathis 0.77 V or higher. If the ORP of the treatment bath is less than 0.77V, even if Fe ions in solution proceed in the manner of Fe²⁺ → Fe³⁺,they are unable to exist in the dissolved state, and oxidized Fe³⁺solidifies. Namely, sludge forms in the phosphate chemical treatmentbath.

[0088] In electrolytic phosphate chemical treatment, a voltage of about10 V or less is preferably applied between the electrodes of thetreatment bath. Namely, when anodic electrolysis is carried out using asteel material for the anode, and cathodic electrolysis is carried outusing an Fe electrode for the anode and an article to be treated for thecathode, Fe dissolves in the treatment bath (Fe → Fe²⁺+2e⁻). Inaddition, when an article to be treated in the form of a steel materialis immersed in a treatment bath at pH 2.5 or lower without applying avoltage, Fe ions dissolve. When a voltage of 10 V or less is appliedbetween the electrodes in the treatment bath, dissolved Fe²⁺ ions arefurther oxidized. Namely, a state exists in the electrolytic treatmentbath in which Fe ions easily proceed in the manner of Fe²⁺ → Fe³⁺. Atthis time, although oxidized Fe ions (Fe³⁺) can be dissolved in thetreatment bath if the ORP (oxidation-reduction potential) of thetreatment bath is 0.77 V or higher, if the ORP is less than 7.70 mV, theoxidized Fe ions (Fe³⁺) are unable to dissolve and solidify. Namely,sludge forms in the treatment bath. Thus, maintaining the ORP(oxidation-reduction potential) of the treatment bath at 0.77 V orhigher is preferable for preventing the formation of sludge andpreventing reaction in the solution phase.

[0089] Next, is a discussion regarding improving the efficiency of themetal surface (electrode interface) reaction. Table 5 shows the mainelementary electrochemical reactions at the electrode interface ofelectrolytic phosphate chemical treatment (in the case of cathodictreatment). A large change in the potential difference occurs at theelectrode interface in electrolytic treatment. Consequently, ions thatreact at the electrode interface undergo a phase transition reactionaccompanying a change in charge. Namely, ions soluble in water become asolid to form a film or become a gas and are removed from the solutionat the electrode interface.

[0090] The reactions of Table 5 are classified in the manner shownbelow.

[0091] (i) Dissolution-precipitation reaction of metal ions

[0092] (ii) Reduction reaction of nitrate ions

[0093] (iii) Decomposition reaction of solvent (water)

[0094] (iv) Dissociation of phosphoric acid and phosphate precipitationreaction

[0095] Furthermore, in the case of using an insoluble anode material incathodic electrolysis, the metal ion dissolution-precipitation reactionof (i) is limited to a precipitation reaction only. Namely, adissolution reaction does not occur in this case.

[0096] The characteristic reactions of electrolytic phosphate chemicaltreatment consist of the nitrate ion reduction reaction of (ii) and thephosphoric acid dissociation and phosphate precipitation reaction. Forthis reason, controlling these two reactions at the electrode interfaceis considered to be an important factor for practical application ofelectrolytic phosphate chemical treatment.

[0097] To begin with, an explanation is provided starting from thenitrate ion reduction reaction. According to Table 5, gas generated inthe reduction reaction of nitrate ions is in the form of N₂O₄, NO₂ orNO. However, as was previously indicated in Table 3, the boiling pointsof N₂O₄ and NO₂ are quite different from NO. When considering the easeof removal of these gases from the treatment bath, it is recommendedthat the gas that is generated be NO because of its low boiling point.TABLE 5 Elementary Electrochemical Reactions at the Electrode Interface(Case of Cathodic Treatment) Anode reactions Cathode reactions Others(i) Metal ion Fe → Fe²⁺ + 2e⁻ Ni²⁺ + 2e⁻ → Ni — dissolution- (−0.44 V)(4) (−0.23 V) (8) precipitation Zn → Zn²⁺ + 2e⁻ Cu⁺ + e⁻ → Cu reaction(−0.77 V) (5) (0.52 V) (9) Ni → Ni²⁺ + 2e⁻ Fe²⁺ + 2e⁻ → Fe (−0.23 V) (6)(−0.44 V) (10) Cu → Cu⁺ + e⁻ Zn²⁺ + 2e⁻ Zn (0.52 V) (7) (−0.77 V) (11)(ii) Nitrate ion — NO₃ ⁻ + 4H⁺ + 3e⁻ → — reduction NO + 2H₂O (0.96 V)reaction (12) NO₃ ⁻ + 2H⁺ + e⁻ → 1/2N₂O₄ + H₂O (0.8 V) (13) (iii) Water2H₂O → O₂ + 4H⁺ + 2H⁺ + 2e⁻ → H₂ — decomposition 4e⁻ (1.23 V) (14) (0 V)(15) reaction (iv) Phosphoric — — H₃PO₄ → 3H⁺ + PO₄ ³⁻ acid (16)dissociation and 2PO₄ ³⁻ + 2Zn²⁺ + Fe²⁺ phosphate → Zn₂Fe(PO₄)₂ (17)precipitation M^(X+)(metal ion) + n(PO₄ ³⁻) → M(PO₄) (18)

[0098] Next, the following is an explanation of a means for obtaining NOas the gas generated in the treatment bath. The respectiveelectrochemical reaction formulas are as follows:

NO³⁻+4H⁺+3e⁻→NO+2H₂O:0.96 V  (12)

NO³⁻+2H⁺+e⁻→1/2N₂O₄+H₂O:0.8 V  (13)

[0099] The electrochemical reaction formulas of formulas (12) and (13)are intended to show that the ORP (oxidation-reduction potential) of thetreatment bath is only equal to or less than the values shown to theright of the formulas, and that the reactions proceed in the directionsof the arrows.

[0100] Namely, this means that, based on formula (13), although the gasthat is generated contains N₂O₄ if the ORP of the treatment bath is 0.8V or lower, if the ORP exceeds 0.8 V, the generated gas can be made toonly contain NO. If the generated gas is only NO, then the effect of thegenerated gas at the electrode surface (interface) is presumed to bemade to be of the same level as the conventional electrolytic surfacetreatment of electroplating. Thus, from the viewpoint of improvingefficiency of the interface reactions, it is preferable to make the ORPof the treatment bath greater than 0.8 V.

[0101] Next, is an explanation of controlling phosphoric aciddissociation and the phosphate precipitation reaction. As was previouslymentioned, it is preferable to maintain the phosphoric acid in solutionas H₃PO₄ in order to not allow the phosphoric acid to react in thesolution phase. In order to accomplish this, the pH is made to be 2.5 orlower. When this is done, phosphoric acid at the electrode interface isdissociated in the manner of H₃PO₄ → PO₄ ³⁻, and a phosphate compound isformed.

[0102] The following summarizes a means for solving the problems of thepresent invention.

[0103] The present invention divides the electrolytic phosphate chemicaltreatment reaction into an electrochemical reaction at the electrodeinterface and an electrochemical reaction in the solution phase, andthen controls each reaction. The present invention is characterized bycarrying out the elementary reactions from a solution to a solid (film)only in the form of an electrochemical reaction at the electrodeinterface. The elementary reactions in which a film is formed from asolution consist of two types of reactions at the cathode interface.These consist of (1) reduction and precipitation reaction of metal ions,and (2) dissociation of phosphoric acid and a precipitation reaction ofphosphate crystals. In order to carry out the two types of reactions atthe cathode interface only, it is necessary to maintain the solutionphase in the state of a solution only. In order to accomplish this, theORP of the treatment bath is maintained at 700 mV or higher, andpreferably 770 mV or higher. Alternatively, in order to more preferablyimprove reaction efficiency and stabilize the treatment bath, the ORP ofthe treatment bath is selected to be 800 mV or higher, and morepreferably 840 mV or higher. The following is a description of aspecific method for maintaining the ORP of the treatment bath at 700 mVor higher. There are two methods for accomplishing this.

[0104] (1) Suppressing (controlling) the amount of Fe electrolysis

[0105] (2) Replenishing and forming Fe-phosphoric acid complex

[0106] The following is an explanation of these methods.

[0107] (1) Suppression (Control) of the Amount of Fe Electrolysis

[0108] Fe ions are recognized to be involved in the film formationreaction in the electrolytic phosphate chemical treatment of the presentinvention. The reasons for Fe ions dissolving in the treatment bathconsist of dissolution in the case the article to be treated in anodictreatment is steel, dissolution from the Fe electrode in cathodictreatment, and dissolution from the Fe electrode when treatment isdormant. Control of the amount of Fe electrolysis from the article to betreated and Fe electrode during treatment can be performed bycontrolling the voltage and current applied. Control of the amounts ofthis electrolysis can be performed if the amount of electrolysis isroughly 0.1 A/dm² or less for both anodic and cathodic electrolysis.

[0109] In addition, the “dormant electrolysis” described in JapaneseUnexamined Patent Publication No. 2000-234200 can be carried out forelectrolysis from the Fe electrode while treatment is dormant.Furthermore, dormant electrolysis refers to suppressing the dissolutionof Fe while treatment is dormant by using a metal that is insoluble inthe treatment bath (such as titanium) for the anode, using an Feelectrode for the cathode, and applying a voltage of 2-5 V.

[0110] (2) Replenishment and Formation of Fe-Phosphoric Acid Complex

[0111] Replenishment and formation of Fe-phosphoric acid complexinvolves replenishment of Fe³⁺ ions in the form of a chemicalpreliminarily in the form of a stable (inactive) complex and not in theform of free (active) ions. The formation of a complex (Fe³⁺−H₃PO₄) byFe³⁺ ions and phosphoric acid is well known. The reactivity of the Fe³⁺ions decreases if a complex is formed. Namely, if the electrochemicalreaction in the solution phase of Fe²⁺ → Fe³⁺+e⁻ (0.77 V) shown in Table4 proceeds, since the solubility of Fe ions differs between Fe²⁺ andFe³⁺, sludge forms if the ORP is lower than 770 mV. The electrochemicalreaction of Fe²⁺ → Fe³⁺+e⁻ (0.77 V) indicates that the reaction can onlyproceed if the applied voltage is 770 mV or higher in the state in whichFe ions are dissolved.

[0112] The addition and dissolution of Fe ions to the treatment bath inthe form of a phosphoric acid complex means that the process of Fe²⁺ →Fe³⁺+e⁻ and its reverse process are omitted simultaneous to free Fe ions(Fe²⁺ or Fe3+) being supplied to the treatment bath (solution phase).Consequently, the treatment bath includes a state in which Fe³⁺dissolved in the form of a complex is in a stable state.

[0113] Preparation of a replenishing liquid containing Fe-phosphoricacid complex is carried out by dissolving iron nitrate in aorthophosphoric acid solution. Actual replenishing liquids also containZn²⁺, Ni²⁺, NO₃ ⁻ and so forth in addition to Fe3+.

[0114] (3) Other Treatment

[0115] The present invention requires that the ORP of the electrolyticphosphate chemical treatment bath be maintained within a suitable rangefor film formation. Reactable treatment bath components of theelectrolytic phosphate chemical treatment bath decrease accompanyingfilm formation. The decrease in reactable components results in adecrease in reactivity and a decrease in the ORP of the treatment bath.Consequently, ORP is adjusted by replenishing the bath with a chemicalcontaining reactable components. For this reason, the ORP of thetreatment bath can be suitably maintained as a general rule bymaintaining a balance between the amount of electrolysis for forming afilm and the replenishment with chemical. Chemical replenishment of thetreatment bath of the present invention is carried out by replenishing achemical having basically the same chemical components as the treatmentbath corresponding to the film that is formed so as to minimizefluctuations in the treatment bath composition according to addition andtreatment of the article to be treated.

[0116] One of the main factors that has an effect on the ORP of thetreatment bath is the pH (hydrogen ion concentration) of the treatmentbath. The pH of a typical replenishing chemical is lower than the pH ofthe treatment bath. Namely, the active hydrogen concentration of thereplenishing chemical is greater. Consequently, when replenishingchemical is added, it tends to act in a direction that lowers the pH ofthe treatment bath, which is turn causes an increase in the ORP of thetreatment bath.

[0117] Consequently, the concentration of active hydrogen ion containedin the replenishing chemical can also be suppressed in order to suppressan increase in the ORP of the treatment bath. More specifically, thedissociated state of H₃PO₄ is controlled even if the composition ofH₃PO₄ contained in the replenishing chemical is the same. Namely,although orthophosphoric acid exists in the equilibrium state ofH₃PO₄/H₂PO₄ ⁻, that state is shifted to the higher concentration ofH₂PO₄ ⁻. The addition of such a replenishing chemical makes it possibleto control increases in the ORP of the treatment bath.

[0118] Continuing, an explanation is provided of the preferable mode formaintaining the ORP of the treatment bath at 840 mV or higher in thepresent invention. In this mode, the filtration and circulation paths ofthe treatment bath are basically open, and as a means of separating theNO₂, N₂O₄ and/or NO gas formed in the treatment bath accompanyingelectrolytic treatment from the treatment bath, by separating thetreatment tank into an electrolytic treatment tank that performselectrolytic treatment and an auxiliary tank that does not performelectrolytic treatment, circulating the treatment bath between the twotanks, and providing a mechanism for exposing the treatment liquid tothe atmosphere, NO₂, N₂O₄ and/or NO gas generated and dissolved in theelectrolytic treatment tank is removed. Namely, in this mode, amechanism is provided that removes nitrogen oxides formed in thetreatment bath accompanying electrolytic treatment in a circulationsystem in which treatment bath subjected to electrolytic treatment inthe electrolytic treatment tank returns to said electrolytic treatmenttank via a circulation pump and filter. This mechanism is basically asystem that opens the filtration and circulation systems of thetreatment bath to the atmosphere.

[0119] In a system in which the filtration and circulation systems areclosed, the treatment bath is in a pressurized state within the system.In the pressurized state, it is difficult for gases dissolved in thetreatment bath to escape from solution. If a mechanism is employed thatopens the filtration and circulation systems to the atmosphere, namely amechanism is employed that reduces pressure, dissolved gases can easilyescape from solution.

[0120] It is preferable to provide a mechanism that is permeable totreatment liquid which allows the passage of membranous and other solidstructures in the above auxiliary tank that does not performelectrolytic treatment, and for example, a filter having a mechanismthat filters treatment liquid is used as the auxiliary tank.

[0121] In particular, a mechanism is provided for the mechanism thatfacilitates escape of gases that extracts a portion of the treatmentliquid prior to being led into a filter cloth or other filtrationmaterial and exposes it to the atmosphere in the above filter. Thetreatment bath is maximally pressurized in front of the filtrationmaterial of the filter. Under these maximally pressurized conditions,gases dissolved in the treatment bath are pushed out of solution andaggregated on the cloth. If a portion of the solution under theseaggregated conditions is extracted and exposed to the atmosphere, theaggregated gases are rapidly released into the atmosphere.

[0122] Furthermore, in the present invention, together with having thefunction of removing sludge, the filter also has the function ofcapturing nitrogen oxide gas (NOx) dissolved in the solution. Thisfunction consists of precipitating dissolved gas (NOx) onto a filtercloth by This action is for allowing the filter cloth to actcatalytically on removal of gas.

[0123] In this manner, by making contrivances in the filtration andcirculation systems, the elementary reactions of electrolytic phosphatechemical treatment differ. The reactions in which NO₃ ⁻ is reduced atthe electrode interface are as shown in (12) and (13) of Table 4.

NO³⁻+4H⁺+3e⁻→NO+2H₂O:0.96 V  (12)

NO³⁻+2H⁺+e⁻→1/2N₂O₄+H₂O:0.8 V  (13)

[0124] Both of these reactions cause the generation of gas from solution(liquid). In addition, when seen from the viewpoint of decomposition ofNO₃ ⁻, N₂O₄ (g) represents the intermediate process of thatdecomposition, while NO (g) represents the final decomposed form.Namely, decomposition of NO₃ ⁻ proceeds in the manner of NO₃ ⁻ → N₂O₄(g) → NO (g). This reduction reaction of NO₃ ⁻ results in an increase involume due to this reaction (from a liquid to a gas). According to LeChatelier's principle, which is one of the basic principles of chemicalreactions, in such a reaction system in which a gas is generated andpressure increases, if the reaction system is set in a direction thatcauses the pressure of the reaction system to decrease, decomposition ofNO₃ ⁻ easily proceeds in the direction of NO₃ ⁻ → N₂O₄ (g) → NO (g).Conversely, if the pressure of the reaction system does not decrease,this indicates that there is the possibility of the decomposition of NO₃⁻ stopping at NO₃ ⁻ → N₂O₄ (g).

[0125] Namely, in the case in which the filtration and circulation pathsof the treatment bath are basically closed systems, decomposition of NO₃⁻ has the possibility of stopping at an intermediate point. Indicatingthis situation in terms of a chemical reaction formula results informula (13) for the decomposition of NO₃ ⁻. This reaction of formula(13) is possible if the ORP of the treatment bath is 800 mv or lower,and consequently, the ORP of the treatment bath is 800 mv or lower.

[0126] In contrast, in the case the filtration and circulation paths ofthe treatment bath are basically an open system, the decompositionreaction of NO₃ ⁻ follows formula (12). In the case the ORP of thetreatment bath is 960 mV or lower, the reaction can proceed according toformula (12). Thus, according to the principle of electrochemicalreactions, in the case the ORP of the treatment bath exceeds 800 mV, thedecomposition reaction of NO₃ ⁻ only proceeds according to formula (12),and by providing a mechanism for venting gas from the lines, that can beeasily achieved. As has been described above, a preferable mode of thepresent invention can be achieved by making the filtration andcirculation system of the treatment bath an open system.

[0127] A preferable mode of the present invention provides a mechanismthat removes NOx gas generated in the treatment bath accompanyingelectrolytic treatment in a circulation system in which treatment bathsubjected to electrolytic treatment in an electrolytic treatment tank isreturned to said electrolytic treatment tank via a circulation pump andfilter. The mechanism that removes NOx gas preferably extracts a portionof the treatment liquid prior to being led into the filtering materialof the filter, exposes it to the atmosphere and removes NOx gas followedby returning it to said treatment tank by a liquid circulation path. Inthis case, the ORP of said treatment bath is made to be 800 mV orhigher, and preferably 840 mV or higher, and gas formed as a result ofdecomposition of NO₃ ⁻ in the treatment bath is preferably only in theform of NO (g).

[0128] Furthermore, the need for maintaining the treatment bath at 840mV or higher originates in formula (19).

NO₃ ⁻2H⁺+2e⁻→NO₂ ⁻+H₂O (0.84 V)  (19)

[0129] The reaction of formula (19) is a reaction that is notaccompanied by a phase transition within the solution phase. Thereaction of formula (19) means that, if the ORP of the treatment bath is840 mV or lower, the possibility exists of NO₃ ⁻ in the solutionchanging to NO₂ ⁻. Such a change in the treatment bath is harmful withrespect to the stability of the treatment bath. For this reason,maintaining the ORP of the treatment bath above 840 mV is preferable forpreventing the reaction of formula (19).

[0130] Although the following provides a more detailed explanation ofthe present invention through its examples, the present invention is notlimited to these examples.

[0131] Examples 1-3 and Comparative Examples 1-2

[0132] The process used in the examples and comparative examples isshown in Table 6. Furthermore, each of the steps of degreasing, rinsing,rinsing, electrolytic phosphate chemical treatment and rinsing arecarried out using a tank having a volume of 200 liters. The degreasingstep is performed by immersing for 4-5 minutes using an alkalinedegreaser at a prescribed concentration and temperature. The rinsingstep is carried out until the degreaser and other chemicals arecompletely removed from the article to be treated. Electrodepositioncoating is performed so that the coated film thickness after baking is15-25 μm, using the Power Top U-56 manufactured by Nippon Paint Co.,Ltd.

[0133] The volume of the electrolytic treatment bath is 200 liters. Thetreatment bath was circulated 6-10 times per hour using a filter toensure the transparency of the treatment bath. In addition, eight setsof automobile air-conditioner parts (clutch, stator housing) used inthis experiment per hanger (treatment jig) were treated in the treatmentbath. This is depicted in FIG. 3. In FIG. 3, reference symbol 1indicates a 200 liter treatment bath, 2 a power supply, 3 an electrode,4 a stator housing (article to be treated), 5 a filter, 6 a pumps, 7 asensor tank (pH electrode, ORP electrode, etc.) and 8 a controller.TABLE 6 Process of Examples and Comparative Examples Electro- lyticphos- phate Steps chemical after De- Rins- treat- chemical Step greasinging Rinsing ment Rinsing treat-ment Examples 0 0 0 0 0 Pure waterCompara- rinsing → tive electro- Examples deposition coating → purewater rinsing → baking (190° C., 25 min.)

[0134] The treatment experiment was performed by immersing the abovehangers containing the 8 sets of articles to be treated in the treatmentbath about every 2.5 minutes and performing electrolytic phosphatechemical treatment continuously for 4 hours. This results in thetreatment of nearly 20 hangers per hour. Furthermore, 2 ml of thechemicals shown in Table 7 were added to the electrolytic reactionsystem of FIG. 3 for each example and comparative example after theinitial treatment and after each treatment of a single hanger. TABLE 7Composition of Replenishing Chemicals (g/Kg, remainder: water) Comp.Comp. Example 1 Example 2 Example 3 Ex. 1 Ex. 2 75% H₃PO₄ 52 52 100 52110 Ni(NO₃)₂ · 400 400 400 628 628 6H₂O Zn(NO₃)₂ · 200 100 100 200 06H₂O ZnO 0 0 25 0 26 Fe(NO₃)₃ · 0 72 0 0 0 9H₂O

[0135] The automobile air-conditioner part (clutch, stator housing)shown in FIG. 4 was used as the article to be treated in the examplesand comparative examples. The stator housing of FIG. 4 consists ofwelding and joining a plate 20 (press stamped part) that serves as aflat surface, and a housing that serves as outer peripheral portion 21(press formed part). The housing serving as the outer peripheral portionis made by deforming a flat plate to an irregular structure by pressforming. For this reason, the outer peripheral portion is a surface thatis greatly deformed in press forming. Lubricating oil strongly adheresto the greatly deformed surface during press forming. This stronglyadhered lubricating oil inhibits the phosphate chemical treatmentreaction. Therefore, this causes a decrease in the performance of thetreated surface (corrosion resistance of the coating). Thus, the outerperipheral portion shown in FIG. 4 decreases in corrosion resistance ofthe coating by non-electrolytic treatment when phosphate chemicaltreatment is performed. This is explained in Japanese Unexamined PatentPublication No. 2000-234200 of the prior art. Electrolytic phosphatechemical treatment is performed in both the examples and comparativeexamples of the invention. The resistance to corrosion of the coating isfavorable in all cases.

[Electrolytic Phosphate Chemical Treatment Method]

[0136] Electrolytic phosphate chemical treatment was carried out withthe electrolysis method shown in FIG. 5.

[0137] The treatment time of electrolytic phosphate chemical treatmentwas 120 seconds. The reason for performing one round of treatment every2.5 minutes was because about 30 seconds were required for movement ofthe hanger and so forth. Electrolytic treatment consisted of cathodicelectrolysis and anodic electrolysis. Cathodic electrolysis consisted ofinitially performing 13 rounds of pulsed electrolysis using an Nielectrode and subsequently performing continuous electrolysis using anNi electrode and Fe electrode. Details of the electrolysis conditions inthe examples and comparative examples are shown in the following table(Table 8). Furthermore, the amount of Fe electrolysis shown in Table 8is the amount of Fe electrolysis when the effective surface area of thearticle to be treated is 2 dm²/piece. TABLE 8 Electro- lysis conditions(per 8 Anodic Cathodic Cathodic hous-ings) electrolysis electrolysis Feelectrolysis Ni Ex. 1 10 V × 0.6 A × Dormant for 42 sec., 1.12 V × 30 Arising for 1 sec., 10 V × 0.6 A × (dormant for 1 holding for 21 risingfor 20 sec., sec., rising for 2 sec. (Amt. of Fe holding for 35 sec.sec.) × 13 times electrolysis: 0.04 (Amt. of Fe 2.10 V × 20 A, A/dm²)electrolysis: 0.04 rising for 15 sec., A/dm²) holding for 43 sec. Ex. 28 V × 0.1 A × Dormant for 42 sec., 1.23 V × 60 A rising for 2 sec., 10 V× 0.0 A × (dormant for 1 holding for 6 sec. rising for 20 sec., sec.,rising for 2 (Amt. of Fe holding for 50 sec. sec.) × 13 timeselectrolysis: 0.0 (Amt. of Fe 2.20 V × 53 A, A/dm²) electrolysis: 0.0rising for 15 sec., A/dm²) holding for 58 sec. Ex. 3 8 V × 0.2 A ×Dormant for 42 sec., 1.10 V × 20 A rising for 1 sec., 8 V × 0.1 A ×rising (dormant for 1 holding for 21 for 20 sec., holding sec., risingfor 2 sec. (Amt. of Fe for 35 sec. (Amt. of sec.) × 13 timeselectrolysis: 0.01 Fe electrolysis: 2.10 V × 17 A, A/dm²) 0.01 A/dm²)rising for 15 sec., holding for 43 sec. Comp. 8 V × 5.1 A × Dormant for42 sec., 1.24 V × 60 A Ex. 1 rising for 2 sec., 18 V × 2.4 A × (dormantfor 1 holding for 6 sec. rising for 20 sec., sec., rising for 2 (Amt. ofFe holding for 50 sec. sec.) × 13 times electrolysis: 0.34 (Amt. of Fe2.18 V × 37 A, A/dm²) electrolysis: 0.15 rising for 15 sec., A/dm²)holding for 58 sec. Comp. 8 V × 2.4 A × Dormant for 42 sec., 1.18 V × 45A Ex. 2 rising for 2 sec., 16 V × 1.1 A × (dormant for 1 holding for 6sec. rising for 20 sec., sec., rising for 2 (Amt. of Fe holding for 50sec. sec.) × 13 times electrolysis: 0.15 (Amt. of Fe 2.16 V × 32 A,A/dm²) electrolysis: 0.07 rising for 15 sec., A/dm²) holding for 58 sec.

[Experiment Results]

[0138] (1) Fluctuations in Treatment Bath Composition andElectrochemical Conditions

[0139] The results of treatment bath composition, chemical analysisvalues and electrochemical conditions accompanying continuouselectrolytic treatment are shown in Table 9.

[0140] Furthermore, the values indicated for ORP in Table 9 are shownbased on an Ag/AgCl electrode serving as the ORP electrode used in theexperiment apparatus. The values indicated with the Ag/AgCl electrodecan be converted to potential values based on the hydrogen standardelectrode potential serving as the indicated value present invention byadding 210 mV to those values. TABLE 9 Treatment bath electrochemicalChem. conditions anal. ORP Ag/ Treatment bath value AgCl composition(g/L) Total elec- Treat- Phos- Nit- Nic- acid- trode ment phate rate kelZinc ity poten- Temp. times ion ion ion ion (Pt) pH tial (° C.) Ex. 1 03.3 21.7 7.3 3.5 28 1.53 616 30.6 20 3.3 21.7 7.2 3.5 28 1.52 597 30.940 3.3 21.7 7.3 3.5 28 1.52 607 31 60 3.3 21.7 7.3 3.5 28 1.51 607 31 803.3 21.7 7.3 3.5 28 1.5 600 31 Ex. 2 0 3.2 11.7 5.1 0.6 18 1.6 625 30.120 3.2 11.7 5.1 0.6 17 1.61 581 31.6 40 3.2 11.7 5.1 0.6 17 1.6 563 31.960 3.2 11.7 5.1 0.6 17 1.62 554 31.6 80 3.2 11.7 5.1 0.6 18 1.62 584 31Ex. 3 0 4.8 16.6 4.6 3.5 25 1.62 627 28.9 20 4.8 16.5 4.6 3.4 25 1.61603 29 40 4.8 16.4 4.7 3.4 25 1.6 586 29.2 60 4.8 16.4 4.6 3.3 25 1.7531 32.5 80 4.8 16.2 4.6 3.3 25 1.69 563 32.7 Comp. 0 3.6 14 6.8 1.6 262.82 256 27.7 Ex. 1 20 3.6 14.1 6.8 1.6 24 2.31 261 31.4 40 3.6 14.1 6.81.6 25 1.98 251 30 60 3.6 14 6.8 1.6 25 2.02 258 29.6 80 3.6 14 6.8 1.625 1.92 267 31.9 Comp. 0 4.2 11.6 4.7 1.4 21 2.02 263 29.6 Ex. 2 20 4.211.5 4.7 1.4 21 1.63 264 31 40 4.2 11.2 4.7 1.4 21 1.64 263 29.5 60 4.211.2 4.7 1.4 21 1.62 267 30.9 80 4.2 11.8 4.7 1.4 21 1.62 268 30

[0141] (2) Evaluation of Coating Corrosion Resistance

[0142] The article to be treated was subjected to electrodepositioncoating in the steps following the chemical treatment of Table 6.Following electrodeposition coating, a coating corrosion resistanceevaluation test was performed on the article to be treated. The coatingcorrosion resistance evaluation test was performed by making scratchesin the coating extending to the substrate with a knife in the flatsurface portion and outer peripheral portion of the article to betreated, and immersing it for 240 hours in a 5% sodium chloride solutionat 55° C. After 240 hours of immersion had elapsed, the article to betreated was rinsed with water and dried by allowing it to stand for atleast 2 hours, followed by affixing adhesive tape to the coated surfacethat was scratched with the knife and then peeling off the tape withconsiderable force. The width of the coating film that was peeled off asa result of peeling off the tape was measured and used to evaluatecoating corrosion resistance. The smaller the peeled width, the betterthe corrosion resistance. The results of evaluation of coating corrosionresistance are shown in Table 10 for both the examples and comparativeexamples. TABLE 10 Results of Evaluation of Coating Corrosion Resistance(peeled width after salt water immersion test, max. value (mm)) Comp.Example 1 Example 2 Example 3 Ex. 1 Comp. Ex. 2 Flat 0 0 0 1 0 surfaceportion Outer 0 1 0 2 0 peripheral portion

[0143] (3) Stability of Treatment Bath

[0144] The stability of the treatment bath (presence of sludgeformation) is shown in Table 11. As indicated in Japanese UnexaminedPatent Publication No. 2000-234200 of the prior art, it is essential inelectrolytic phosphate chemical treatment that the treatment bath betransparent during treatment. The formation of sludge was not observedin the treatment bath during treatment for any of the examples andcomparative examples. Thus, coating corrosion resistance was alsosatisfactory. However, when the treatment bath was allowed to stand for3 days following completion of continuous treatment, sludge formed inthe treatment baths of the comparative examples. There was no formationof sludge in the treatment baths of the examples. The treatment baths ofthe comparative examples both had an ORP of about 260 mV (Ag/AgClelectrode), and this is equivalent to a potential based on the hydrogenstandard potential of about 470 mV, which does not fall within thepresent invention. TABLE 11 Comp. Example 1 Example 2 Example 3 Ex. 1Comp. Ex. 2 During None None None None None treatment 3 days None NoneNone Formed Formed after treatment

[Explanation of Examples 1-3 and Comparative Examples 1-2 and Analysisof Experiment Results] Example 1:

[0145] Example 1 is the standard method of the present invention. Theamount of Fe electrolysis is controlled and the standard chemical isused. For this reason, there is no formation of sludge in the treatmentbath even after standing.

Example 2:

[0146] Example 2 is an example of the present invention in the case ofusing a replenishing chemical containing Fe ions.

Example 3:

[0147] Example 3 is an example of the present invention showing the useof a chemical in which the degree of dissociation of phosphoric acid hasbeen adjusted for the replenishing chemical in order to lower the ORP ofthe treatment bath. Furthermore, the same chemical as in Example 1 isused starting in the 61st round of treatment in Example 3. This is donefor the purpose of raising the ORP again after it has lowered.

Comparative Example 1:

[0148] Comparative Example 1 is an example of an increased amount of Feelectrolysis and a lowered ORP of the treatment bath. The amount of Feelectrolysis is 0.15 A/dm², which is larger than that of the examples.

Comparative Example 2:

[0149] In comparative example 2, although the amount of Fe electrolysisis large at 0.15 A/dm² with respect to anodic electrolysis, that withrespect to cathodic electrolysis is suitable at 0.07 A/dm². In thisexample, however, the chemical used for the replenishing chemical inwhich the degree of dissociation of phosphoric acid is adjusted is thesame as that used in Example 3. When the use of a chemical in which thedegree of dissociation of phosphoric acid is adjusted is continued, theORP of the treatment bath lowers.

Examples 4 and 5

[0150] These examples are examples of mass production equipment thatform a filtration and circulation circuit in which the tank volume is1000 liters, the filter volume 400 liters and the total volume of thetreatment bath, including the volume of the sensor tank and so forth, is1500 liters. The filtration and circulation path is an open systemcomposed with lines as shown in FIG. 6 (Example 4) or a closed systemcomposed with lines as shown in FIG. 7 (Example 5). In FIGS. 6 and 7,reference symbol 9 indicates a hanger, 10 a filter cloth, and 11 anarticle to be treated. In the open system of FIG. 6, in addition to maincirculation line 12, pressure reducing open line 13 is also provided.Gas dissolved in the solution is discharged from this pressure reducingopen line 13. These steps are basically as described in Table 6 (withthe exception of two degreasing steps), and each step is carried out bya series of equipment in a tank having a volume of 1000 liters. In eachstep, the article to be treated is immersed for 110 seconds and thenmoved to the next step in 40 seconds. An alkaline degreaser at aprescribed concentration and temperature is used in the degreasingsteps. The electrolytic treatment bath is circulated 3-12 times per hourwith a filtration circulation pump. The treatment hanger is used duringtreatment by attaching 30 automobile air-conditioner parts in the formof the article to be treated shown in FIG. 4 per side, or 60 parts onboth sides, to each hanger. Electrodeposition coating is performed sothat the coated film thickness after baking is 15-25 μm, using the PowerTop U-56 manufactured by Nippon Paint Co., Ltd.

[0151] Although the basic constitution of the electrolytic phosphatechemical treatment apparatus is as shown in FIG. 3, the volume ischanged as previously described. Eight Ni electrodes and two Feelectrodes are provided for film forming electrodes. Four Ni electrodeseach are installed on both sides of the hanger so that current flowsuniformly to the article to be treated. In addition, one Fe electrodeeach in the form of an iron core having a diameter of 10 mm is installedon both sides of the hanger. The treatment bath was made to be able tocirculate through the treatment tank 3-12 times per hour via the filter.In addition, the chemicals shown in Table 12 were added to theelectrolytic treatment reaction bath at 62 mL/hanger in Example 4 and 30mL/hanger in Example 5 for each hanger treated. TABLE 12 Example 4Example 5 H₃PO₄ 85 g/L 115 g/L  NO₃ 296 g/L  270 g/L  Ni 80 g/L 50 g/LZn 68 g/L 85 g/L

[0152] Electrolytic phosphate chemical treatment was carried outaccording to the method of FIG. 5. This treatment was performed for 110seconds/cycle-hanger, after which the hanger was moved to the next tankin 40 seconds. Thus, treatment for 110 seconds was repeated every 150seconds. Electrolytic treatment was carried out in the order of anodicelectrolysis followed by cathodic electrolysis. Cathodic electrolysisconsisted of initially performing 8 rounds of pulsed electrolysis usingan Ni electrode and subsequently performing continuous electrolysisusing an Ni electrode and Fe electrode. The details of theseelectrolysis conditions are shown in Table 13. TABLE 13 Electrolysisconditions (per Cathodic Cathodic 60 work Anodic electrolysiselectrolysis pieces) electrolysis Fe Ni Examples 4 5 V × 0.1 A × Dormantfor 42 (1) 8.5 V × 200 A and 5 rising for 1 sec., sec., 4 V × 0.1(dormant for 1 holding for 8 sec. A × rising for 20 sec., rising for 2sec., holding for sec.) × 8 times 35 sec. (2) 8.5 V × 200 A, rising for15 sec., holding for 43 sec.

[Experiment Results]

[0153] (1) Treatment Bath Composition and Electrochemical Conditions

[0154] The mean results of treatment bath composition, chemical analysisvalues and electrochemical conditions in the case of continuouselectrolytic treatment with mass production equipment are shown in Table14.

[0155] Furthermore, the values indicated for ORP in Table 14 are shownbased on an Ag/AgCl electrode serving as the ORP electrode used in theexperiment apparatus. The values indicated with the Ag/AgCl electrodecan be converted to potential values based on the hydrogen standardelectrode potential serving as the indicated value of the presentinvention by adding 210 mv to those values. TABLE 14 Status of PhosphateChemical Treatment Bath (Mean Values) Treatment bath electrochemicalChem. conditions anal. ORP Ag/ Treatment bath value AgCl composition(g/L) Total elec- Phos- Nit- Nic- acid- trode phate rate kel Zinc itypoten- Temp. ion ion ion ion (Pt) pH tial (° C.) Ex.4 12.2 46 17.1 14.186 1.23 674 30 Ex.5 7.69 31.5 12 8.99 54 2.48 597 33

[0156] In Example 5, the pH was higher, ORP was lower and concentrationsof treatment bath components were lower than in Example 4. Thisindicates that the filtration-circulation system is a closed system, andthat the electrochemical reaction efficiency is inferior to an opensystem. The ORP of 597 mV indicates the possibility of the occurrence ofthe reaction of formula (19), which is one of the reactions in thesolution phase (solution reaction) in the treatment bath. The potentialbased on the Ag/AgCl electrode of the reaction of formula (19) is 630 mVor less.

NO₃ ⁻+2H⁺+2e^(−→NO) ₂ ⁻+H₂O (0.84 V)  (19)

[0157] If the reaction of formula (19) actually occurred, the componentsin solution would react and the solution state would tend to break down.Consequently, a solution state would result that facilitated theformation of sludge, the stability of the treatment bath as a solutionwould decrease, and the both allowed to stand would form sludge easily.In actuality, sludge formed when the bath was allowed to stand for 3days. On the basis of this, making the filtration-circulation circuit ofthe treatment bath an open system and removing NOx that forms sludgewere confirmed to be preferable for the stability of the treatment bath.

[0158] (2) Evaluation of Coating Corrosion Resistance

[0159] The article to be treated was subjected to electrodepositioncoating in the steps following the chemical treatment previouslydescribed. Following electrodeposition coating, a coating corrosionresistance evaluation test was performed on the article to be treated.The coating corrosion resistance evaluation test was performed in thesame manner as the test method in Examples 1-3. The results are shown inTable 15. TABLE 15 Example 4 Example 5 Flat surface portion 0 0 Outerperipheral portion 1 1

[0160] (3) Evaluation of Coating Adhesion

[0161] Following electrodeposition coating, a coating adhesionevaluation test was performed on the article to be treated. Evaluationof coating adhesion was performed according to the cross-cut adhesionmethod of JIS-K 5400 8.5.1 using a gap interval between cuts of 1 mm or2 mm. Cuts were made in the flat surface portion at a gap interval of 1mm, while cuts were made in the inner peripheral portion at a gapinterval of 2 mm. The reason for using a gap interval of 2 mm for thecuts made in the inner peripheral portion was because current flowseasier through the inside of the work piece (inner peripheral portion)than through the outside (flat surface portion), and it was difficult tomake cuts at a gap interval of 1 mm. Those results are shown in Table16. TABLE 16 Example 4 Example 5 Flat surface portion 0% 0% Innerperipheral portion 0% 10% 

[0162] (4) Stability of Treatment Bath

[0163] The stability of the treatment bath is shown in Table 17. Theformation of sludge was not observed in the treatment bath duringtreatment in Example 4 or 5. However, as was previously mentioned, whenthe treatment bath was allowed to stand for 3 days following completionof continuous treatment, sludge formed in the treatment bath of Example5. There was no formation of sludge in the treatment bath of Example 4.The treatment bath of Example 5 had an ORP of 597 mV (Ag/AgClelectrode), and although this is equivalent to a potential based on thehydrogen standard potential of about 807 mV, since there was no removalof NOx in this case, Example 4, which was accompanied by NOx removaltreatment, was indicated as being preferable. TABLE 17 Example 4 Example5 During treatment None None 3 days after treatment None Formed

[Explanation of Examples 4 and 5 and Analysis of Experiment Results]

[0164] Examples 4 and 5 are examples of practical mass productionsystems. When applied to mass production equipment, it was confirmedthat it is preferable to make different accommodations than those ofExamples 1-3 using experimental systems. Namely, since the treatmentvolume is continuous and large, the removal of NOx gas, which was ableto be ignored in the experimental systems, is important. The differencebetween Examples 4 and 5 is the presence or absence of removal of NOxgas. This difference between the two was manifest in their respectivetreatment baths. Namely, if NOx gas is not removed, the concentration ofNOx gas in the treatment bath does not decrease, and this acts in thedirection of inhibiting the reduction reaction of NO₃ ⁻, with thereaction of formula (19) coming to act as the solution reaction.

NO₃ ⁻+2H⁺+2e^(−→NO) ₂ ⁻+H₂O (0.84 V)  (19)

[0165] Consequently, the electrolysis reaction efficiency in thetreatment bath decreases. As a result, since chemical components are notconsumed, the component concentration of the treatment bath increases,the stability of the treatment bath as a solution decreases, andsusceptibility to sludge formation increases. Moreover, if theelectrolysis reaction efficiency decreases, the adhesion of the coatingfilm as well as coating corrosion resistance also decrease. Therefore,removal of NOx gas was found to be preferable particularly in cases inwhich the treatment volume is not small, but rather large andcontinuous.

[0166] According to the present invention, the following effects aredemonstrated.

[0167] (1) Substantial Elimination of Sludge Formation in the TreatmentBath

[0168] The present invention was shown in principle to be able tosubstantially eliminate sludge. However, in the case of actual massproduction equipment, variations exist in the contents of the treatmentbath. In order to reduce variations in the reactions and treatment bath,the ORP in the treatment bath should be raised and maintained at 840 mVor higher. If this is done, sludge formation can be substantiallyreduced to zero, with the exception of minor variations.

[0169] (2) Improvement of Chemical Film Quality

[0170] In the present invention, the electrochemical reactionsaccompanying phase transitions relating to film formation can be limitedto only electrochemical reactions at the electrode interface bysubstantially eliminating sludge formation. In addition, thedecomposition reaction of N₃ ⁻ at the electrode interface can be made toconsist only of formula (12), thereby making it possible to improveelectrolysis reaction efficiency. Consequently, the film that is formedcan be formed reliably adhered to the article to be treated. For thisreason, in the case of a coating substrate, a film can be formed havinga coating corrosion resistance superior to cases in which sludge isformed.

What is claimed is:
 1. An electrolytic phosphate chemical treatmentmethod of forming a film composed of a phosphate compound and a metalthat is reduced and precipitated from an ionic state on the surface of ametal material article to be treated by performing electrolytictreatment on said article to be treated in a phosphate chemicaltreatment bath by contacting said metal material having electricalconductivity with said phosphate chemical treatment bath containingphosphate ions and phosphoric acid, nitrate ions, metal ions that form acomplex with phosphate ions in said phosphate chemical treatment bath,and metal ions for which the dissolution-precipitation equilibriumpotential at which ions dissolved in said phosphate chemical treatmentbath are reduced and precipitate as metal is equal to or greater than−830 mV, which is the cathodic reaction decomposition potential of thesolvent in the form of water when indicated as the hydrogen standardelectrode potential, and is substantially free of metal ions other thanthose which are a component of the film; wherein, the ORP(oxidation-reduction potential) of said phosphate chemical treatmentbath (indicated as the potential relative to a standard hydrogenelectrode) is maintained at equal to or greater than 700 mV.
 2. Anelectrolytic phosphate chemical treatment method according to claim 1,wherein said electrolytic treatment preferably uses for the electrodematerial that dissolves in the treatment bath a metal that forms acomplex with phosphoric acid and phosphate ions in the phosphatechemical treatment bath and/or a metal material for which thedissolution-precipitation equilibrium potential at which ions dissolvedin the phosphate chemical treatment bath are reduced and precipitate asmetal is greater than or equal to −830 mV, which is the cathodicreaction decomposition potential of the solvent in the form of waterwhen indicated as the hydrogen standard electrode potential, and a metalmaterial that is insoluble during electrolysis.
 3. The electrolyticphosphate chemical treatment method according to either claim 1, whereinthe amount of Fe ions dissolved into the treatment bath from an Feelectrode, when performing cathodic treatment of said article to betreated and using an Fe electrode as the electrode that dissolves in thetreatment bath, is controlled in order to make said ORP of the phosphatechemical treatment bath equal to or greater than 700 mV.
 4. Theelectrolytic phosphate chemical treatment method according to claim 1,wherein in the case the article to be treated is a steel material, theamount of Fe ions dissolved into the treatment bath in anodic treatmentin which said steel material in the form of the article to be treated isdissolved as the anode, and the amount of Fe ions that dissolve in thetreatment bath from an Fe electrode in cathodic treatment, arecontrolled so that the ORP of the phosphate chemical treatment bath isequal to or greater than 700 mV.
 5. The electrolytic phosphate chemicaltreatment method according to claim 1, wherein the electrode used inelectrolysis for making the ORP of the phosphate chemical treatment bathequal to or greater than 700 mV is an insoluble metal material.
 6. Theelectrolytic phosphate chemical treatment method according to claim 1,wherein a chemical that contains Fe ions which replenishes the phosphatechemical treatment bath is an Fe-phosphate complex in order to make theORP of said phosphate chemical treatment bath equal to or greater than700 mV.
 7. The electrolytic phosphate chemical treatment methodaccording to claim 1, wherein the ORP of the phosphate chemicaltreatment bath is equal to or greater than 770 mV.
 8. The electrolyticphosphate chemical treatment method according to claim 1, wherein metalions that form a complex with phosphoric acid and phosphate ions in thephosphate chemical treatment bath are preferably at least one type ofZn, Fe, Mn or Ca ions.
 9. The electrolytic phosphate chemical treatmentmethod according to claim 1, wherein NO, NO₂ and/or N₂O₄ gases generatedand dissolved in an electrolytic treatment tank are removed from thetreatment bath by separating the treatment tank into an electrolytictreatment tank that carries out electrolytic treatment and an auxiliarytank that does not carry out electrolytic treatment, circulating thetreatment bath between the two tanks, and providing a mechanism thatopens treatment liquid to the atmosphere either between the above twotanks or within the two tanks, as a means of separating NO₂, N₂O₄ and/orNO gas formed in the treatment bath accompanying electrolytic treatmentfrom the treatment bath.
 10. The electrolytic phosphate chemicaltreatment method according to claim 9, wherein the auxiliary tank thatdoes not carry out electrolytic treatment has a mechanism in which thetreatment liquid is passed through a permeable solid structure.
 11. Theelectrolytic phosphate chemical treatment method according to claim 10,wherein the solid structure is a film.
 12. The electrolytic phosphatechemical treatment method according to claim 9, wherein a filter havinga mechanism that filters the treatment liquid is used for the auxiliarytank that does not carry out electrolytic treatment.
 13. Theelectrolytic phosphate chemical treatment method according to claim 9,having a liquid circulation circuit that removes a portion of thetreatment liquid at a location prior to being introduced into a filtermaterial in a filter, exposes the removed treatment liquid to theatmosphere, and returns it to the electrolytic treatment tank afterremoving gases in the form of nitrogen oxides present in the treatmentliquid.
 14. The electrolytic phosphate chemical treatment methodaccording to claim 9, wherein the ORP of the treatment bath is equal toor greater than 840 mV.
 15. The electrolytic phosphate chemicaltreatment method according to claim 9, wherein the treatment bath ismaintained in a constant state by measuring the above ORP value of thetreatment bath and changing the amount and/or composition ofreplenishing chemical corresponding to the change in that value.